Q&A
Stereo Preamplifier
Balance
50k
Bass
50k
Treble
50k
47k
47k
47k
+12V
Off
0.22
0.22
0.22
Loudness
47k
Volume
On
12
79
14
4
17
50k
11
0.22
0.47
LM1036
Rin
2
8
Rout
0.47
Lin
19
13 Lout
13
+
47uF .01
5 6 15
+
10uF 0.39
16 18
+
0.39 10uF .01
10, 20
Figure 4
and triacs.
Needs Stereo
Chips
QI would like to find an
inexpensive preamp or
integrated amp ( 20 watts or
so) to place in a really nice
cabinet that matches some
other equipment. I need one
that has volume, bass, treble, and balance. They can
be separate modules from a
number of companies. It
seems to me that there used
to be a lot of these things
around in kit form, but
several hours of surfing on
the Web proved fruitless.
I’m willing to build it from
scratch using transistors
or ICs.
be replaced with a metal-oxide resistor (MOV). An MOV behaves like
back-to-back zeners and can be used
in both DC and AC circuits (see
Figure 2). The choice usually boils
down to available space and cost.
An ideal suppression voltage is 10
volts; that is, the back EMF should
be held at 10 volts, with 15 volts as
an upper limit.
In addition to arc suppression
across relay contacts is the minimization of electromagnetic interference (EMI). EMI can wreck havoc
with RF communications and spark
the turn-on of logic gates, SCRs,
Bruce Brown
via Internet
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NUTS & VOLTS
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ANational Semiconductor makes
a nice line of power amp ICs
called the Overture series. They
range in power from 20 to 60 watts
and are housed in multi-pin TO-220
plastic packages. The amplifiers are safeguarded at the
output against overvoltage,
undervoltage, overloads, thermal runaway, and instantaneous temperature peaks.
The LM1876 matches your
request for a power amp.
Figure 3 shows the schematic
for the power amp section.
An LM1036 audio preamp — also from National
Semiconductor — provides
tone, volume, and balance.
Figure 4 shows the schematic
for the preamp section. A
loudness switch has been
included to modify the frequency curve as the volume
level is changed. Notice that
the bass and treble controls
act on both channels. If you
want separate tone controls
for each channel, you will
need two LM1036 chips,
APRIL 2005
+12V
Figure 5
10k
1k
84
220k
7
555
63
2
15
1uF .01 1k
RR Crossing Bell
+12V
LED Flasher
84
10k 100k
.01
3
555
.01
+
4001
4001
4001
7
6
2
Cross
Bell
1uF 1 5
.01
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